1. Field of the Invention
The present invention relates to a cold forging steel excellent in grain coarsening prevention and delayed fracture resistance and a method of producing the same.
2. Description of the Related Art
Cold forging (including roll-forging) is utilized for bolts, gear components, shafts and numerous other products because it enables fabrication of products with excellent surface quality and dimensional precision, is lower in cost than hot forging, and is excellent in yield. In the cold forging of such products, use is made of medium-carbon machine structural carbon steels and alloy steels such as those specified by S G 4051, JIS G 4052, JIS G 4104, JIS G 4105, JIS G 4106 and the like. The process usually includes a step of annealing or spheroidization annealing before the cold forging, in the manner of, for example: hot rolling--annealing--cold forging--quench-hardening--tempering. This is because the high as-rolled hardness of medium-carbon carbon steels and alloy steels like those listed above is a cause of various production-related problems, including high cost owing to heavy wear of the cold forging tool during the shaping of components such as bolts and occurrence of cracking during component shaping owing to the low ductility of the blank.
As annealing involves considerable energy, labor and equipment costs, however, a need is felt for a material and process that enable omission of the annealing step. This has led to the development of numerous so-called low-carbon boron steels that enable omission of the annealing step by reducing the carbon and alloying element content of the steel to achieve lower as-hot-rolled hardness and improved ductility and that add a small amount of boron to make up for the degradation in quench-hardening performance caused by the reduced content of Cr, Mo and other alloying elements. Such steels are taught by, for example, JP-A-(unexamined published Japanese patent application)5-339676, JP-B-(examined published Japanese patent application)5-63624 and JP-A-61-253347. Although addition of a small amount of boron (B) improves the quench-hardening performance, this effect is lost when N is present in the steel in solid solution because the B combines with N to form BN. Ordinarily, therefore, Ti is added to fix the N in the steel as TiN and thereby suppress formation of BN.
As the need for components with higher strength has increased, attempts have been made to apply such low-carbon boron steels to higher strength components. Since low-carbon boron steels are low in C and alloying elements, however, they sustain a decline in delayed fracture property when subjected to heat treatment for achieving a tensile strength of 1000 MPa or higher. It is known that an attempt to obtain high strength by conducting low-temperature tempering results in degraded delayed fracture properties. However, when the amount of added C is increased or an SCR, SCM or other such alloy steel is used in order to secure high strength and bring the delayed fracture strength up to a practical level even with high-temperature tempering, the resulting increase in the steel hardness makes it impossible to eliminate the annealing step. Although low-carbon boron steels that enable omission of annealing are economical, they require the tempering temperature to be lowered for obtaining high strength. But this degrades the delayed fracture strength and causes problems from the practical aspect. Application to high-strength products is therefore difficult.
In response to the call for application of boron steels to high-strength components, JP-A-8-60245, for example, teaches a steel reduced in impurity content so has to have delayed fracture property on a par with an alloy steel. When this boron steel was evaluated using a machined-surface test piece, it was in fact found to exhibit a delayed fracture property superior to an alloy steel. However, when the steel was used to fabricate a component on an actual production line, and the delayed fracture property was evaluated from the heat-treated surface condition, it was found that the boron steel component was inferior to an alloy steel in delayed fracture property. The technology taught by JP-A-8-60245 is therefore limited in its ability to respond to the need for higher strength components.
In addition to the foregoing problems, a boron steel is also more likely than an annealed steel to sustain abnormal coarsening of specific austenite grains during heating for quench-hardening. A component that has experienced grain coarsening is liable to have low dimensional precision owing to quench-hardening distortion, reduced impact value and fatigue life, and, particularly in a high-strength component, degraded delayed fracture property. Application of a boron steel to a high-strength component therefore requires suppression of grain coarsening and crystal grain refinement. For suppressing the grain coarsening, it is effective to finely disperse a large quantity of particles that pin grain boundary movement.
Methods have been proposed for preventing the aforesaid grain coarsening of boron steel. JP-A-61-217553, for example, aims to pin the grain boundaries by defining the Ti and N contents as 0.02&lt;Ti-3.42N so as to generate TiC. However, it is not possible to prevent grain coarsening merely by defining composition because the TiC cannot be finely dispersed. On the other hand, JP-B-63-64495, for instance, aims to prevent grain coarsening by keeping N content to a very low value of not greater than 0.0035% and subjecting the resulting composition having an excess of Ti relative to N to rolling under low-temperature heating. However, prevention of grain coarsening cannot be achieved unless the TiC, Ti(CN) precipitation condition is optimized before heating for quench-hardening.
JP-A-52-114545, for example, puts TiC into solid solution at the material stage so that fine precipitation of TiC will first occur during heating for quench-hardening. When pinning particles precipitate during heating for quench-hardening, however, the amount of TiC precipitation is affected by the heating rate during heating for quench-hardening or heating for carburization. As this makes the expression of the pinning effect unstable and, even when the same material is used, a high probability arises of the coarsening prevention being degraded by a mere change in component size or the heat-treatment furnace. A problem therefore persists regarding quality stability in actual production.
The aforesaid conventional methods cannot achieve a delayed fracture property of the actual component equal to or better than that of an alloy steel when the annealing or spheroidization annealing step before cold forging is omitted and heat treatment is conducted for imparting high strength.